Due to the hostile environment in the cylinders, it is a challenge to retrieve necessary information about the combustion process cycle-by-cycle. Without such information, it is impossible to optimize the overall engine efficiency and stability and to minimize emissions.
Control of Otto engines basically amounts to controlling three primary variables: ignition timing and fuel and air injected into the cylinder. For the two latter both the mass and the timing are important and these are controlled separately using different actuators such as the throttle, the fuel injectors, and the intake valves depending on engine design and mode of operation. For Diesel engines the main control variables are timing and mass of injected fuel. The main actuators for diesel engine control are, consequently, the fuel injectors. In today's engine control systems, most of the control functionality is implemented inform of look-up tables, which give the optimal ignition timing, say, for a certain operating point of the engine and at certain prevailing ambient condition. These systems require extensive calibration tests to meet the performance requirements under all driving conditions, including varying speed and load, fuel quality, air temperature, air pressure, air humidity, etc. Calibration of an engine management system is therefore typically a very time consuming and expensive task and, sometimes, the result is not satisfactory. There is a need for supplements to look-up tables in order to enable more efficient control of the engine.
It has been suggested to use continuous, i.e., cycle by cycle, measurements of the combustion conditions (combustion feedback signal) in order to eliminate the need of extensive calibration. Ionisation current measurements and in-cylinder pressure measurements are two possible ways of obtaining the desired information (combustion feedback signal) for engine control, as is known from, e.g., SE-504197. The combustion feedback signal can be measured either directly in the combustion chamber, (as is known per se from e.g., R. Muller, M. Hart, A. Truscott, A. Noble, G. Rrotz, M Eickhoff, C. Cavalloni, and M. Gnielka, “Combustion Pressure Based Engine Management System”, SAE paper no. 2000-01-0928, 2000; J. Auzins, H. Johansson, and J Nytomt, “Ion-gap sense in misfire detection, knock and engine control”, SAE paper no. 950004, 1995) or indirectly using non-intrusive sensors (as is known per se from, e.g., M. Schmidt, F. Kimmich, H. Straky, and R. Isermann, “Combustion Supervision by Evaluating the Crankshaft Speed and Acceleration”, SAE paper no. 2000-01-0558, 2000; M. Sellnau, F. Matekunas, P. Battiston, C-F. Chang, and D. Lancaster, “Cylinder-Pressure-Based Engine Control Using Pressure-Ratio-Management and Low-Cost Non-Intrusive Cylinder Pressure Sensors”, SAE paper no. 2000-01-0932, 2000). As described in said publications (and publications defined below), these measurements can be used for closed-loop engine control and enable realtime optimisation with respect to desired features such as fuel consumption, emissions, power and stability. Also, the measurements can be used for misfire and knock detection, individual cylinder air/fuel ratio control, camshaft phasing, control of start-of-combustion, EGR rate control, etc. See, e.g., Muller et al. (2000); Sellnau et al. (2000) according to the above, or H. Wilstermann, A. Greiner, P Hohner, R. Kemmler, R. Maly, and J. Schenk, “Ignition System Integrated AC Ion Current Sensing for Robust and Reliable Online Engine Control”, SAE paper no. 2000-01-0553, 2000; or L. Nielsen and L. Eriksson, “An Ion-Sense Engine Fine-Tuner”, IEEE Control Systems, 1998.
In order for an engine control system to operate correctly in a closed loop, it must have sufficient and accurate combustion process related information. However, this information is hard to retrieve due to the hostile environment. Moreover, the interrelation between the combustion parameters may be very complex and therefore extremely difficult to handle in both open and closed-loop control systems. If, for instance, the fuel/air mixture is changed of some reason, then the burn rate will change, which in turn leads to a change in the peak pressure position that is used for closed-loop ignition timing control (e.g. SE 504 197). This leads to a sub optimization, which results in decreased efficiency of the engine and higher emission levels. The root cause to this problem is that only a subset, if any, of the combustion parameter values of interest are available to the engine control system. The invention alleviates these problems by providing robust and accurate combustion parameter estimates cycle by cycle.
There are several combustion parameter estimates that are necessary in a closed-loop control system in order to enable a better performance of the engine. Some examples follow.
Peak Pressure Location (PPL) is a parameter that describes the location of the crankshaft, i.e. the crank angle degree, when the pressure in the cylinder is at its maximum. The engine has an optimal performance when the peak pressure is achieved at a certain crank angle. If the PPL diverges from the optimal value, it is advantageous to adjust PPL so as to return to the optimal value. This can be done in several ways, for example by changing the ignition time or the air/fuel ratio.
Mass Fraction Burnt (MFB) is a parameter, which indicates at which crank angle degree a certain amount of the fuel mixture has been combusted. This parameter is strongly correlated to PPL.
Air Fuel Ratio (AFR) is a parameter stating the ratio between air and fuel in the mixture. The performance of an engine depends of the AFR and the optimal value varies with temperature, humidity and other factors. Therefore it is important to control and measure the AFR in order to control the engine for optimal performance. If AFR can be measured individually for each cylinder, then it is possible to balance each cylinder, thus achieving an optimal AFR for each cylinder. Slate of the art is to measure the AFR using a lambda sensor in the exhaust manifold, i.e., the mean of the AFR in the cylinders connected to the manifold is measured. In this case cylinder balancing with respect to AFR is not possible.
Knock is a parameter that depicts when non-combusted fuel self ignites due to increased pressure and temperature. When the fuel mixture is ignited and a flame front is spread from the sparking plug, the pressure and temperature increases drastically and a knock may be initialized. Knocking combustions are uncontrolled and large pressure peaks, which are harmful to the engine, may occur. Knock can be avoided by advancing the spark timing. However, this reduces the performance of the engine. To achieve maximum performance from an engine it is often preferable to run close to the knock limit, which is dependent of the fuel quality (fuel grade). By knock detection the engine control system can control the engine to work at optimal performance without passing the knock limit.
Misfire is when the fuel mixture fails to ignite. Law regulates the amount of allowed misfires in an engine since non-combusted gases are harmful to both the environment and the car catalyst.
Combustion Stability is a parameter relating to the stability of the combustion process from cycle to cycle. Large variations in the engine combustion can be perceived as a non-smooth jerky performance and is therefore undesirable.
Torque is of interest, especially when using automatic gears. It is preferred to have zero “moment” from the engine during shift of gears.
In conventional engine control the engine is commonly calibrated in an experimental environment by the use of high quality measurement probes such as sophisticated lambda and cylinder pressure sensors. The calibration data are thereafter used to create look-up tables from which the engine management system reads the parameter settings for different engine work conditions, e.g. spark timing and AFR setting for a given RPM and work load. A problem with this approach is that it does not solve real-time problems such as varying fuel quality and air humidity and wearing of the engine or individual differences between the cylinders. Hence the engine tends to run on non-optimal engine settings. If the combustion parameters could be estimated in real-time with high enough accuracy and robustness, then the engine control could work in a closed loop and these problems would be alleviated.
The combustion parameters mentioned above can to some extent be measured using different probes that are dedicated to the specific application, e.g., a lambda sensor to measure the AFR, a piezo-electric vibration sensor mounted on the engine to measure knock and measurements of the crank axis acceleration in order to detect misfires. There are sensors, however, that can be used to estimate all of the above mentioned combustion parameters.
Pressure probes are often used during development of an engine and measures the pressure directly in the cylinder. However, pressure probes are expensive and have a short life span and have therefore not yet been used in serial production.
Ion-current sensor systems are alternatives to pressure sensors. When the fuel mixture is combusted, electrons and ions are formed which make the gas conductive, i.e., it achieves the ability to carry an electric current. The concentration of charged particles in the combusted gas depends on the pressure and temperature in the cylinder. Hence, by applying a voltage over the spark plug and measure the resulting current, information about the combustion process can be retrieved. Through ion-current data analysis it is possible to estimate all combustion parameters, but hither ion-current measurement has only been used for estimation of knock and misfire when used in serial production due to the stochastic nature of ion-current. There is a potential to improve this technology considerably using the signal processing proposed in the present innovation.
Known strategies for estimation of combustion parameters from, e.g., ion-current measurements can be divided into two main categories. The first category consists of algorithms that estimate the combustion parameters by looking for characteristic “phenomenon” in the combustion measurements that correlates to a reference measurement. Such phenomena can be a maximum, an inflexion point or other criteria in the data. However, this approach is difficult to apply over all working conditions of the engine since the type of phenomenon that correlates with the combustion parameter may depend on the actual workload conditions of the engine (REM, load, etc.). Hence, a local maximum can be of interest during a certain workload condition, but in another workload condition an inflexion point is more suitable. Therefore, this approach in reality encounters considerable practical problems and hence the usability becomes limited. In this approach no a priori knowledge such as the parameter probability distribution or a signal model is utilized.
The second approach is to use a deterministic signal model that describes the combustion process. The model is then parameterized by a set of parameters that are estimated from the data. An example of such a model that has been applied to ion-current measurements is a sum of “Gaussian bulbs” (e.g Se 504 197). The model is fitted to the data in a least square sense with respect to the parameters, thus yielding model parameter estimates. The combustion parameters are then derived from the estimated Gaussian model. So, for example, the AFR can be estimated from the slope of the first gauss-curve and the PPL can be estimated from the top of the second gauss-curve. This approach uses a priori knowledge of the combustion process in the form of a deterministic signal model. However, the match between the data and the model limits the success of this approach. So, if the model does not have the ability to accurately describe the data, then the quality of the estimated combustion parameters will not be adequate. This will be the case when the model structure is not correct or the degree of freedom in the model is not high enough. Unfortunately, to, find a model that accurately describes the data in all practical engine operation points is very hard.
US 2002/0078930 describes a control device controlling an engine where the AFR can be changed in accordance to the running environment. In WO 96/05419 a method and a system for adaptive correction of the amount of fuel supplied to two-stroke combustion engines. U.S. Pat. No. 6,505,500 describes an arrangement for detecting ionization in the combustion chamber of a combustion motor where the fuel self-ignites by means of compression, as well as associated measurement device and calibration devises. U.S. Pat. No. 6,526,954 describes a system for regulating me fuel-air mixture m internal combustion engine. The system utilizes binary sensors to detect relative deviations from stoichiometric combustion, including individual combustion events, and allows for regulation to achieve optimal and similar combustion to take place in all the cylinders. In CA2281621 a controller receives the ionization signal and controls the air/fuel ratio in the engine based at least in part upon the ionization signal. In a preferred embodiment of the control system, the controller controls the air/fuel ratio based upon a first local peak in the ionization signal. In another embodiment, the controller controls the air/fuel ratio based upon maximizing the first local peak in the ionization signal.